1. Field of the Invention
The present invention relates generally to a complementary metal-oxide semiconductor (CMOS) xylophone bar magnetometer with automatic resonance control.
2. Description of the Related Art
A Lorentz force-driven mechanical resonator measures the deflection in a conducting bar produced by the Lorentz force as represented by the equationF=I×B  (1)where F is the Lorentz force, I is a current, and B is a magnetic field. FIG. 1 illustrates an embodiment of a Lorentz force-driven mechanical resonator in the form of a xylophone bar magnetometer (XBM) 5, which is described in commonly owned U.S. Pat. No. 5,959,452, issued Sep. 28, 1999, the contents of which are incorporated herein by reference. The XBM 5 is comprised of a resonator 10, in this case a thin conductive bar, e.g., aluminum, supported by two wires 12 and 14. The wires 12 and 14 are bonded to the resonator 10 to provide low-resistance electrical contacts and are positioned at the nodal points expected for a bar free at both ends and vibrating in its fundamental mode.
In operation, alternating currents, generated by a sinusoidal source oscillating at the fundamental transverse resonant mode, are supplied to the resonator 10 at one of two support nodes 16 and extracted at the other node 18, and the device is placed inside a magnetic field. The Lorentz force generated by the current and the applied magnetic field causes the bar to vibrate in its fundamental mode, the vibrational amplitude being proportional to the vector component of the magnetic field parallel to the support wires in the plane of the bar.
The amplitude of the vibration can be measured using various techniques, including optical beam deflection, optical interferometry, and differential capacitance and tunneling currents. The Lorentz force-driven mechanical resonator structure can serve as a fundamental component for numerous RF applications.
FIG. 2 illustrates a Lorentz force-driven mechanical resonator based mixer/filter component 20 that provides a basis for an RF-mixer/filter array design, which is described in commonly owned international application, Ser. No. PCT/US02/13058, filed Apr. 24, 2002 and published under international publication number 02/088764 on Nov. 7, 2002, the contents of which are incorporated herein by reference. A local oscillator (LO) input signal at a frequency FLO drives a pair of magnetic field coils 22 to create a magnetic field (B). In this design, the magnetic field coils 22 are placed lengthwise on either side of Lorentz force-driven mechanical resonator 26. An RF input signal at frequency fur passes through an impedance matching network 24 and drives an electrical current (I) in the mechanical resonator 26. If the RF frequency is equal to (FLO+f0) or to (FLO−f0), where f0 is the resonance frequency of the mechanical resonator 26, then the mechanical resonator 26 begins to resonate. A pair of support arms 28 supports the mechanical resonator 26. The ends of one support arm 28 are coupled with anchor/electrodes 30 that receive the impedance matched RF input signal while the ends of the other support arm 28 are coupled with anchor/electrodes 30 that are grounded. A readout electrode 32 is coupled with the mechanical resonator 26 to provide a Lorentz force output signal (F) for the mixer/filter component 20. In this design, the amplitude of the vibration of the mechanical resonator 26 is determined via direct measurement of capacitance between the bar and an electrode 32 placed near the bar. Other methods or means for determining the amplitude of the vibration of the mechanical resonator 26 may be substituted.
In its implementation as a mixer/filter, a Lorentz force-driven mechanical resonator is a component that can be fashioned into a combined mixer/IF filter for traditional superheterodyne receiver applications, as illustrated in FIG. 2. Because of its high mechanical Q factor, the Lorentz force-driven mechanical resonator can eliminate the multiple conversion stages required in traditional superheterodyne receivers that operate in the UHF to VHF range. To achieve a narrow-IF bandwidth though traditional means, the IF frequency must be relatively low compared to the bandwidth of the signal of interest due to the limitations on the Q factors of electronic devices. For many practical applications, this necessitates the use of multiple IF stages within a receiver system. However, the Lorentz force-driven mechanical resonator allows for high IF frequencies with very high Q values reducing the requirements of the image reject filter while also supplying high compression of interfering signals.
The magnetometer, sensor, and other electronics are manufactured on a single die using a standard CMOS process. The magnetometer, as described above, is a vibrating bar structure with a resonant frequency that is determined by its dimensions and the position of two anchors attached to a CMOS substrate. The Lorentz force causes the bar to vibrate in the presence of a magnetic field vector when an alternating current is present between the anchors of the xylophone (one anchor is a current source, the other is a sink), and the frequency of the current is equal to the resonant frequency of the bar. The magnitude of the vibration is proportional to the magnitude of the magnetic field vector, given a current source with constant amplitude.
A number of conducting fingers protrude from the xylophone bar in the direction perpendicular to the direction of motion. Matching fingers are interdigitated in the static portion of the CMOS die to which the anchors are attached. These interdigitated fingers create a capacitor when the xylophone bar is at rest. As the bar moves in a direction perpendicular to the CMOS substrate, the capacitance formed by these fingers changes from its resting (nominal) value. Exploiting this feature, a capacitive sensor can be used to determine the amount of deflection in the xylophone bar, and hence determine the magnitude of any magnetic field vector that is present.